(i)Digital Subscriber Line (DSL)
DSL is a family of technologies that use ordinary analog telephone lines to provide digital data transmissions. It uses different frequencies for voice and data signals, and the same telephone line can simultaneously be used for phone and data transfer. It is commonly used for high-speed Internet access from homes and offices. Different DSL technologies are collectively noted as x DSL and support data transfer speeds from 128 Kbps to 24 Mbps, as discussed in the following list:
Asymmetrical DSL (ADSL)
ADSL is the most common of all types of DSL variations. The download speed of data is faster than upload speeds. It uses one channel for analog voice (telephone) transmissions, a second for data uploads, and a third for data downloads.
Symmetrical DSL (SDSL)
SDSL supports equal speeds for both data uploads and downloads. It cannot be used for voice transmissions and hence is suitable only for Internet access at offices.
ISDN DSL (IDSL)
IDSL is a variation of symmetric DSL. It does not support analog voice trans- missions and is used only in those environments where ADSL and SDSL are not available.
Rate Adaptive DSL (RADSL)
RADSL is a variation of asymmetric DSL that can vary the transfer speeds depending on line conditions. It supports both data and voice transmissions.
High Data Rate DSL (HDSL)
HDSL is a variation of asymmetric DSL that uses twisted copper wires. It supports both data and voice transmissions.
Very High Data Rate DSL (VHDSL)
VHDSL is a symmetric variation of DSL that supports high-speed transmissions. It does not support sharing the line with voice signals.
DSL is a family of technologies that use ordinary analog telephone lines to provide digital data transmissions. It uses different frequencies for voice and data signals, and the same telephone line can simultaneously be used for phone and data transfer. It is commonly used for high-speed Internet access from homes and offices. Different DSL technologies are collectively noted as x DSL and support data transfer speeds from 128 Kbps to 24 Mbps, as discussed in the following list:
Asymmetrical DSL (ADSL)
ADSL is the most common of all types of DSL variations. The download speed of data is faster than upload speeds. It uses one channel for analog voice (telephone) transmissions, a second for data uploads, and a third for data downloads.
Symmetrical DSL (SDSL)
SDSL supports equal speeds for both data uploads and downloads. It cannot be used for voice transmissions and hence is suitable only for Internet access at offices.
ISDN DSL (IDSL)
IDSL is a variation of symmetric DSL. It does not support analog voice trans- missions and is used only in those environments where ADSL and SDSL are not available.
Rate Adaptive DSL (RADSL)
RADSL is a variation of asymmetric DSL that can vary the transfer speeds depending on line conditions. It supports both data and voice transmissions.
High Data Rate DSL (HDSL)
HDSL is a variation of asymmetric DSL that uses twisted copper wires. It supports both data and voice transmissions.
Very High Data Rate DSL (VHDSL)
VHDSL is a symmetric variation of DSL that supports high-speed transmissions. It does not support sharing the line with voice signals.
(ii)Broadband cable
Broadband Internet Access, or simply Broadband, is provided by the cable companies that provide digital cable services. It is a reliable and efficient means of Internet access. Access is provided through a cable modem that further connects to the computer or to other network devices. Low-cost wired or wireless routers are commonly used to share a single broadband connection among several computers in a home or in small offices.
With a cable modem, the user does not have to dial the ISP, and the connection is always live. This might pose a security risk for computers that are used for critical purposes. Most cable modems support bandwidths from 1.5 to 3 Mbps for Internet access. The cable modem usually supports up to 10 Mbps data speeds for the LAN. The actual Internet access speed depends on the utilization of the shared cable signals in the area. The available bandwidth is always shared with other users in the area and may vary from time to time. In the periods of peak usage, the speed may be low compared to the periods when usage is low. Both broadband and baseband are signaling technologies.
Broadband Internet Access, or simply Broadband, is provided by the cable companies that provide digital cable services. It is a reliable and efficient means of Internet access. Access is provided through a cable modem that further connects to the computer or to other network devices. Low-cost wired or wireless routers are commonly used to share a single broadband connection among several computers in a home or in small offices.
With a cable modem, the user does not have to dial the ISP, and the connection is always live. This might pose a security risk for computers that are used for critical purposes. Most cable modems support bandwidths from 1.5 to 3 Mbps for Internet access. The cable modem usually supports up to 10 Mbps data speeds for the LAN. The actual Internet access speed depends on the utilization of the shared cable signals in the area. The available bandwidth is always shared with other users in the area and may vary from time to time. In the periods of peak usage, the speed may be low compared to the periods when usage is low. Both broadband and baseband are signaling technologies.
(iii)Plain Old Telephone System/Public Switched Telephone Network (POTS/PSTN)
POTS and PSTN are the traditional methods of Internet access. These are dial-up methods; the user has to dial the telephone number of the ISP to authenticate and get Internet connectivity. The telephone line is connected to a modem that is further connected to a serial or USB port of the user’s computer. Most computers have built-in modems that can be directly connected to the telephone line. In case the model is connected to an external port such as the serial or the USB port, its software driver must also be installed.
POTS and PSTN provide a maximum data transfer speed of 56 Kbps. There are several ISPs that offer dial-up Internet access. Depending on the area in which the user lives, one must be careful while selecting the ISP. Most ISPs provide added features, such as free email accounts and access to newsgroups, and some even offer small web site for the user.
POTS and PSTN are the traditional methods of Internet access. These are dial-up methods; the user has to dial the telephone number of the ISP to authenticate and get Internet connectivity. The telephone line is connected to a modem that is further connected to a serial or USB port of the user’s computer. Most computers have built-in modems that can be directly connected to the telephone line. In case the model is connected to an external port such as the serial or the USB port, its software driver must also be installed.
POTS and PSTN provide a maximum data transfer speed of 56 Kbps. There are several ISPs that offer dial-up Internet access. Depending on the area in which the user lives, one must be careful while selecting the ISP. Most ISPs provide added features, such as free email accounts and access to newsgroups, and some even offer small web site for the user.
(iv)Satellite
In such areas where DSL or cable is not available, satellite Internet is the only option for high-speed Internet access. For this reason, it is commonly used in rural areas. The signals travel from the ISP to a satellite and then from the satellite to the user. The data transmission speeds vary from 512 Kbps (upload) to 2 Mbps (download). Major drawbacks of satellite Internet access are that it is expensive, and it offers low transfer speeds compared to DSL and cable.
Satellite Internet access suffers from propagation delays or latency problems. Latency refers to the time taken for the signal to travel from the ISP to the satellite and back to the user. The signals have to travel to a satellite located in the geostationary orbit that is about 35,000 Km away. This means that the signals have to travel approximately 70,000 Km before they reach the user. Latency also depends on atmospheric conditions. This might be a problem for businesses or home users that rely on real-time applications.
In such areas where DSL or cable is not available, satellite Internet is the only option for high-speed Internet access. For this reason, it is commonly used in rural areas. The signals travel from the ISP to a satellite and then from the satellite to the user. The data transmission speeds vary from 512 Kbps (upload) to 2 Mbps (download). Major drawbacks of satellite Internet access are that it is expensive, and it offers low transfer speeds compared to DSL and cable.
Satellite Internet access suffers from propagation delays or latency problems. Latency refers to the time taken for the signal to travel from the ISP to the satellite and back to the user. The signals have to travel to a satellite located in the geostationary orbit that is about 35,000 Km away. This means that the signals have to travel approximately 70,000 Km before they reach the user. Latency also depends on atmospheric conditions. This might be a problem for businesses or home users that rely on real-time applications.
(v)Wireless
Wireless. Wireless networks rely on radio frequencies to communicate instead of network cabling used for normal computer networks. Radio frequencies create electromagnetic (EM) fields, which become the medium to transfer signals from one computer to another. As you go away from the hub, or the main equipment generating the radio frequency of the wireless network, the strength of the EM field reduces and the signal becomes weak.
Wireless networks defined in IEEE 802.11 standards use radio frequencies with spread spectrum technology. The two spread spectrum technologies are as follows:
Frequency-hopping spread spectrum (FHSS)
This is the method of transmitting RF signals by rapidly switching frequencies according to a pseudorandom pattern, which is known to both the sender and the receiver. FHSS uses a large range of frequency (83.5 MHz.) and is highly resistant to noise and interference.
Direct-sequence spread spectrum (DSSS)
This is a modulation technique used by wireless networks, which uses a wide band of frequency. It divides the signal into smaller parts and then transmits them simultaneously on as many frequencies as possible. DSSS is faster than FHSS and ensures data protection. It utilizes a frequency range from 2.4 GHz to 2.4835 GHz and is used in 802.11b networks.
The most popular of the IEEE 802.11 wireless network standards are 802.11b, 802.11a, and 802.11g. Table 21 gives a brief comparison of the characteristics of different 802.11 standards.
Wireless. Wireless networks rely on radio frequencies to communicate instead of network cabling used for normal computer networks. Radio frequencies create electromagnetic (EM) fields, which become the medium to transfer signals from one computer to another. As you go away from the hub, or the main equipment generating the radio frequency of the wireless network, the strength of the EM field reduces and the signal becomes weak.
Wireless networks defined in IEEE 802.11 standards use radio frequencies with spread spectrum technology. The two spread spectrum technologies are as follows:
Frequency-hopping spread spectrum (FHSS)
This is the method of transmitting RF signals by rapidly switching frequencies according to a pseudorandom pattern, which is known to both the sender and the receiver. FHSS uses a large range of frequency (83.5 MHz.) and is highly resistant to noise and interference.
Direct-sequence spread spectrum (DSSS)
This is a modulation technique used by wireless networks, which uses a wide band of frequency. It divides the signal into smaller parts and then transmits them simultaneously on as many frequencies as possible. DSSS is faster than FHSS and ensures data protection. It utilizes a frequency range from 2.4 GHz to 2.4835 GHz and is used in 802.11b networks.
The most popular of the IEEE 802.11 wireless network standards are 802.11b, 802.11a, and 802.11g. Table 21 gives a brief comparison of the characteristics of different 802.11 standards.
No comments:
Post a Comment